It is often desirable to determine the position on a surface with respect to a reference point thereon, such position often being expressed in two coordinates. For example, it is often desirable to determine the position on a planar surface with respect to a reference point of origin so that the position can be expressed in two coordinates, e.g., an X-coordinate and a Y-coordinate.
Such position determinations are required in graphical data devices wherein the position of a stylus with respect to a fixed set of reference coordinates is determined as the stylus is moved over the reference surface. Such devices, or writing tablets as they are often known, permit the user to digitize graphic points and supply their data content directly into storage for display on a screen of a data processing terminal.
An example of such a coordinate digitizing device is described in U.S. Pat. No. 3,648,277, issued to A. L. Whetstone on Mar. 7, 1972, which device utilizes a localized source of magnetic energy positioned with respect to a wire array, each of the wires of the array terminating in a common delay line. Such wire matrices are also utilized in other systems of a similar nature as described in U.S. Pat. No. 3,767,858 issued to J. L. Rodgers on Oct. 23, 1973; U.S. Pat. No. 3,851,097 issued to J. L. Rodgers on Nov. 26, 1974; and U.S. Pat. No. 3,886,311 issued to J. L. Rodgers et al. on May 27, 1975. In all such systems a grid array, or matrix, of separate wires is utilized adjacent, or embedded within, a surface on which a position is to be determined. Accordingly, the resolution and accuracy of such position determination is limited by the spacing of the wires. Moreover, the use of separate wires in a matrix form requires extremely precise positioning of such wires giving rise to an increase in the difficulties and costs of manufacture thereof, such problems being particularly pronounced in the manufacture of miniature two coordinate positioning devices requiring very high resolution.
Another presently available device has been made and sold by Tektronix, Inc. as Graphic Tablet Model No. 4953/54 and utilizes a matrix of wires in much the same manner as discussed in the above issued patents.
In view of the resolution and accuracy limitations on such devices, it is desirable to provide a device in which continuous position determination can be achieved with a much higher degree of resolution utilizing a structure which eliminates the need for a complex matrix of individual wires which must be accurately aligned in the X and Y-coordinate directions at high manufacturing costs.
Another approach is described in U.S. Pat. No. 3,838,212 issued to A. L. Whetstone et al. on Sept. 24, 1974, the device therein using a stylus which generates a spark for producing a fast rise time, sound energy shock wave at the location of the stylus. The shock wave propagates through the air and receiver devices positioned along the X and Y coordinates respond to the leading edge of the air-propagated shock wave to provide elapsed time indication for determining the X and Y coordinates.
U.S. Pat. No. 3,156,766 issued to G. M. Stamps on Nov. 10, 1964 discloses a sonar telecriber which utilizes a glass platen and requires a stylus transducer in contact with the surface thereof at a position to be determined. Arrays of piezoelectric crystals in contact with the edges of the platen detect the arrival of a mechanical pulse signal generated at the stylus to determine such position from the times of travel of the mechanical pulse signal to each array. U.S. Pat. No. 3,134,899 issued on May 19, 1964 to P. W. Woo depicts a similar system in which the piezoelectric crystal arrays are each energized by a pulse signal and the arrival of the mechanical pulse signals induced thereby in the platen at a detector transducer located in contact with the platen surface at the position to be determined is detected. The times of travel of such mechanical pulse signals then permit the position of the detector transducer to be determined.
Both the Stamps and Woo systems require the use of mechanically induced pulse signals which affect the resolution and accuracy of the position determinations due to the mechanical limitations of the geometry of the stylus and the surface irregularity of the platen. Moreover, the pressure of the contact of the stylus transducer will tend to induce multimode mechanically induced pulse signals in the platen the detection of which will be difficult for short delays. Moreover, it is not possible to induce a single longitudinal mode, continuous wave signal in the glass platen in either the Stamps or Woo systems so that the ability of such systems to provide low dispersion pulses is severely hampered. Further, the continuous contact of the moving stylus on the platen surface tends to wear out the surface and affect the capability of inducing "clean" mechanical pulse signals in the material. The costs of such systems appear to be relatively high since a large number of piezoelectric transducers are required therein, such transducers also requiring operation in the same electrical phase, which requirement raises a difficult problem in the manufacture and assembly process thereof.
Furthermore, the mechanically induced signals require a direct contact between the stylus and the platen. Therefore, in applications where it is desirable to digitize a pattern present on a material which has low acoustical conductivity, such as a rubber sheet or carpet, for example, it is not possible to induce the mechanical waves when the rubber sheet is placed on the platen between the stylus and the platen surface.